The present invention relates to a new and improved priority valve for controlling fluid flow from a pump to an auxiliary apparatus and a hydrostatic steering controller.
A known priority valve for controlling the flow of fluid to a hydrostatic steering controller and to an auxiliary apparatus is disclosed in U.S. patent application Ser. No. 243,497, filed Mar. 13, 1981 by Jim L. Rau and Ronald L. LaHue and entitled "Hydrostatic Load-Sense Steering System". This application discloses a priority valve which responds to changes in the demand for fluid by the steering controller and to changes in the steering load. The priority valve disclosed in the aforementioned application includes a priority flow control orifice through which fluid flow to a hydrostatic steering controller is varied as a function of variations in the demand for fluid flow and as a function of variations in the steering load. The priority valve also has a single variable size orifice through which fluid which is not required for steering purposes is directed to an auxiliary apparatus. To meet the demand for fluid flow, the priority valve seeks to achieve a steady state condition in which a constant pressure differential is maintained across a variable orifice located in the steering controller.
The priority valve disclosed in U.S. application Ser. No. 243,497 includes a valve spool which is biased by the pressure derived from a pilot flow of fluid into a position in which the main flow of hydraulic fluid is directed to a steering controller. Specifically, a pilot flow of fluid is established through the hydrostatic steering controller. The pilot flow is restricted when a steering operation is initiated. The flow restriction causes a back pressure to develop in the pilot line which acts on a priority valve spool to move the spool into a position to direct fluid flow to the steering controller. This priority valve is satisfactory in its mode of operation and is effective to respond quickly to changes in the demand for fluid by the power steering system. Nonetheless, it would be desirable to improve the valve so that it could supply demands for steering fluid more smoothly and accurately despite a wide range of fluid flows to the valve from a pump or other source of fluid. Such an improved valve would have to provide a consistent steady state pressure differential across the variable orifice in the steering controller and rapidly regain the pressure differential after responding to rapid variations in steering and auxiliary flow requirements and the flow and pressure from the pump. An improved valve would also have to hold to a minimum the pressure drop across the valve, from its inlet to its auxiliary port.
There are two well recognized ways of enabling a priority valve to satisfy widely varying demands for steering fluid and maintain a minimum bypass pressure drop across the valve. One way is to provide the valve spool with a relatively large diameter land. A relatively small axial movement of the land relative to a fixed valve surface will thus open an annular orifice between the land and the valve surface which has a relatively large maximum cross sectional area. Unfortunately, when the spool must control relatively small fluid flows, difficulty is encountered in accurately moving the valve spool through the small distances required to meter the flow and maintain the desired pressure differential across the variable orifice in the steering controller. Further, a valve spool with a large diameter land is heavy and has a high inertia. The biasing spring that acts on the spool must exert a higher preload, which may require a higher spring rate. A higher spring rate will make it more difficult to maintain the desired pressure differential. A large priority valve also requires a relatively large pilot flow of fluid when the valve is used in a system such as shown in the aforementioned U.S. patent application Ser. No. 243,497. Thus, providing a valve spool with a large diameter land may be ineffective to maintain a desired pressure differential, although flow could be bypassed to auxiliary at an appropriate rated pressure drop.
The other well known way of increasing the flow capacity of a priority valve is to move the valve spool axially through a larger distance. Moving a valve spool through a larger axial distance rather than increasing the diameter of its lands, will permit the valve to control accurately relatively small fluid flows. At the same time, however, the increased axial spool movement will make the valve under-responsive at high fluid flows. Moreover, when the valve spool is moved through relatively large distances, the variations in spring rate caused by compression and extension of the spool's biasing spring will be increased. Changes in the spring rate will tend to cause wider variations in the desired pressure differential. Thus, moving a valve spool through a longer axial distance may also be ineffective to maintain a desired pressure differential.
With the known ways of permitting a valve to handle large and small fluid flows, the response of the valve to changing operating conditions is sluggish or under-responsive. The valve cannot quickly or smoothly respond to rapid increases or decreases in the quantity of fluid required by the hydraulic circuits to which the valve is controlling flow. The ability of the valve to control its desired constant pressure differential is diminished in order to obtain a low pressure drop across the valve to the auxiliary port.